Hyperlink `#123` to the respective issue/ticket

Adding this pull request in response to Issue #187. 

I added this a very long time ago to another fork of GitX. Now I rebased this commit onto GitX-dev master.

Here is how it works, just add `gitx.ticketurl` to your repositorys `.git/config`.
```
[gitx]
   ticketurl = "http://trac.example.org/ticket/*" # note: '*' gets replaced by the number 
```

To point to github issues just use:
```
$ git config gitx.ticketurl 'https://github.com/rowanj/gitx/issues/*'
```